|By Abdi Sheikh and Feisal Omar

|By Abdi Sheikh and Feisal Omar

|By Abdi Sheikh and Feisal Omar

MOGADISHU (Reuters) - Suicide bombers killed at least 13 people at the gates of the African Union's main peacekeeping base in the Somali capital on Tuesday, police said, in an attack claimed by the Islamist militants of al Shabaab.

The force of the bombings shattered windows at Mogadishu's nearby airport, showered arriving passengers with glass and forced the suspension of flights, police and witnesses said.

The United States condemned the attack on the AMISOM peacekeeping force's base, the latest by the militant group that wants to topple Somalia's western-backed government and rule the Horn of Africa country according to sharia law.

"At least 13 people - mostly security forces - died in the two car bomb blasts," and 12 others were wounded, police officer Abdiqadir Omar told Reuters. Al Shabaab, which is linked to al Qaeda, said it set off two car bombs.

Police said the first attacker detonated a car bomb and the second tried to storm the base on foot, but was shot and exploded at the gate. The guards were caught in the blast as they escorted U.N. personnel into the base, known as Halane.

In a statement, White House spokesman Ned Price said the U.S. stood with Somalia in its fight against "despicable acts of terrorism that seek to destabilize" the country.

"We remain committed to helping Somalia progress along a path towards peace and prosperity and the defeat of terrorist groups, including al-Shabaab,” he said.

AMISOM said on Twitter it condemned the "senseless attacks that aim to disrupt and cripple the lives of ordinary Somalis". There was no immediate comment from the United Nations.

People arriving on international flights said the blasts shattered windows in the airport buildings.

"We were greeted by two loud blasts. The glass of the airport building fell on us," said Ali Nur, who had just got off a plane from Nairobi.

Al Shabaab regularly attacks AMISOM, which is made up of about 22,000 military personnel from Uganda, Kenya, Ethiopia and other African countries helping to support Somalia's government and army.

Somalia was plunged into anarchy in the early 1990s following the toppling of military dictator Mohamed Siad Barre.

The U.N. refugee agency on Tuesday increased its funding appeal to nearly $500 million to finance the voluntary return and reintegration of Somali refugees from the sprawling Dadaab camp in Kenya, which hosts some 330,000 Somalis.

Kenya, citing security threats, said last month it aims to reduce by almost half the population of the camp.

"Despite the security situation currently in Somalia, people are returning on a daily basis," UNHCR's representative in Somalia Caroline van Buren told a news briefing in Geneva.

"We cannot assure the refugees that they will be safe, but we take precautions," she said. "For instance, when we have return convoys, we inform AMISOM of the convoys, the routes they will be taking, the number of vehicles, the number of people. It is not 100 percent, but we are doing the best we can."

(Additional reporting by Stephanie Nebehay in Geneva and Jeff Mason in Washington; Writing by Duncan Miriri and Elias Biryabarema; Editing by Tom Heneghan)